CSFB is specified today in the 3rd Generation Partnership Project (3GPP) Technical Specification (TS) 23.272 (see, e.g., V12.2.0 of March 2014). In brief, CSFB permits in an Evolved Packet System (EPS) the provisioning of voice and other CS domain services (such as services pertaining to Unstructured Supplementary Service Data, or USSD) by re-use of CS infrastructure when the UE is served by the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN). A CSFB-enabled UE, connected to E-UTRAN, may thus use 2nd or 3rd Generation (2G or 3G) technologies such as GSM Edge RAN (GERAN) or UTRAN to connect to the CS domain.
The mechanism for handling a terminating call needed to be modified in certain CSFB scenarios. Specifically, CSFB is only available in case E-UTRAN coverage (as defined by Tracking Areas, or TAs) is overlapped by either GERAN or UTRAN coverage (as defined by, for example, Location Areas, or LAs). A general problem results from the fact that there exists no 1:1 mapping between TAs and LAs.
For a terminating call the lacking congruency between TAs and LAs may have the consequence that the UE, when falling back from E-UTRAN to GERAN or UTRAN, may land in a LA that is not controlled by the “old” MSC-S towards which the SGs interface association for the UE has been established by the MME. In such a case the “old” MSC-S will not be able to terminate the call.
Mobile Terminating Roaming Forwarding (MTRF) is a procedure that has been specified in 3GPP TS 23.018 (V12.2.0 September 2013), in order to deliver mobile terminating calls to subscribers that change MSC area during the setup of a terminating call. The MTRF procedure may be applied for CS calls towards mobile subscribers roaming in GERAN or UTRAN access, but also towards mobile subscribers performing CS fallback from E-UTRAN to GERAN or UTRAN, as specified in 3GPP TS 23.272.
In addition, 3GPP specifies the concept of IMS centralized services (ICS) in TS 23.292, TS 24.292 and TS 29.292. ICS allows IMS subscribers to get services executed in IMS when they are connected via CS radio access. One way to deploy ICS is to enhance the MSC with ICS support, and such an MSC is called ‘MSC server enhanced for ICS’ in the 3GPP standards. The MSC Server enhanced for ICS is connected with the IMS domain via a so called I2 interface.
When the UE registers in the MSC Server enhanced for ICS via a CS Location Update procedure and the subscriber is an ICS subscriber (for example determined by an indication received from the HLR as part of the subscriber data), the MSC Server enhanced for ICS shall register the subscriber in IMS.
When the MSC has registered the ICS subscriber in IMS, then IMS may route terminating calls (voice or video call) for that subscriber directly to that MSC based on the information received at the IMS Registration procedure, as defined in 3GPP TS 23.292 for IMS calls.
This means that for terminating calls from IMS no GMSC and no related roaming number retrieval procedure is applied. But the current solutions specified for MTRF do not cope for the case that the MSC is enhanced for ICS and that the user is an ICS user.